Use of an oxide ceramic material for compression molds for shaping elements made from glass or a glass-containing ceramic and having high surface quality and dimensional accuracy

ABSTRACT

The use is described of oxide ceramic materials for compression molds for shaping elements made from glass or a glass-containing ceramic and having high surface quality and tight tolerances. The materials are polycrystalline, monophasal or polyphasal materials which are contact-inert towards the optical elements to be pressed and which, in addition, have significantly better mechanical and thermal properties than monocrystalline substances, which, in principle, exhibit an anisotropic behavior as a consequence of their geometrical anisotropy. The material intended for use according to the invention can comprise a Al 2  O 3  ceramic or a ZrO 2  -- and/or HfO 2  -containing ceramic. The material can also be built up in several phases, i.e. can contain a matrix component and at least one particle component dispersed discretely in it. They are preferably ZrO 2  -containing ZTC, PSZ or TZP ceramics, which, for the application according to the invention, have excellent thermomechanical properties which are founded in the reaction mechanism and, above all, in the crystallography of their components. Mechanisms such as so-called &#34;stress-induced conversion toughening&#34; and &#34;microcrack toughening&#34; play an important part here. 
     With the materials used, compression molds can be produced for reworking-free shaping of optical elements having planar and/or spherical and/or aspherical convex and/or concave surfaces of high surface quality and dimensional accuracy, while noting improved production processes.

The invention relates to the use of inorganic, nonmetallic, oxideceramic materials as compression molds for the production of elementsmade from glass and/or a glass-containing ceramic and having highquality and dimensional accuracy. As a consequence of their shapeaccuracy and surface quality, the optical elements thus produced neednot be subjected to further, expensive processing, such as grinding,polishing etc.

PCT/DE 84/00,128 has already disclosed compression molds for opticalelements which comprise monocrystalline, nonmetallic, inorganicmaterials, such as, for example, NiO, Cr₂ O₃, sapphire or spinell.Although these materials are suitable, in particular, for use at hightemperatures with respect to their contact inertness, considerabledifficulties arise during the production of the compression molds andduring the pressing processes themselves carried out therewith as aconsequence of the anisotropic character of the materials and as aconsequence of their strength behavior.

Monocrystalline substances, ie. so-called "monocrystals", exhibitanisotropy in their physical, chemical, mechanical and thermalproperties; ie. the properties mentioned here change as a function ofthe crystallographic orientation present in each case.

Thus, for example, the hardness, and thus the abrasion resistance, ofmonocrystals varies depending on the orientation. This state of affairsalone leads to various erosion rates during polishing, so that theprecision of shaping, which is in the nanometer region in the case ofsuch molds, can only be achieved with difficulty.

Since the mold must be heated to high temperatures during the actualpressing process, a material expansion caused by the heat takes place.In the case of monocrystalline materials, this is again dependent on thecrystallographic orientation of the pertinent monocrystal. This meansthat the geometry of the compression mold which is present at roomtemperature changes in a fashion such that the optical elements to beshaped do not have the desired or necessary optical precision fit. Thishas the consequence that complicated mathematical computationaloperations for retrocorrection of the mold become necessary. A furtherdisadvantage of monocrystalline materials is that the production of aspatial shape, for example a dish shape, can only be carried out if thecrystal has previously been aligned precisely with respect to itscrystal axes using complicated light or x-ray optical methods andclamped.

In addition to the heat-expansion anisotropy to be found in noncubicmonocrystals, anisotropy of the elastic constants, which, like thehardness anisotropy discussed above and the thermal conductivityanisotropy discussed below, also applies to maximum-symmetry cubicmonocrystals, presents difficulties, since the pressing process,logically, proceeds under pressure and crystal orientation-dependentelastic expansion or contraction must also be taken into account.

Anisotropy of the therml conductivity has a negative effect in thatlocal overheating occurs both in the compression mold and in the pressedpart itself, thus causing thermally induced mechanical stresses andbuckling.

With respect to the strength behavior, monocrystals prove to beextremely brittle. The so-called critical stress intensity factor K_(ic)can be regarded as a measure of the brittleness, where

    K.sub.ic =σ.√a.f[MN/m.sup.3/2 ]

in this formula,

σ: denotes the externally applied tensile stress on;

a: denotes the length of a crack perpendicular to the external tensilestress, and

f: denotes a geometry factor.

For example, this value for monocrystalline aluminum oxide is merely:

    K.sub.ic (Al.sub.2 O.sub.3)=2 MNm.sup.-3/2.

For the use of monocrystal molds, this principally means an increaseddanger of fracture under mechanical and thermal load.

The object of the present invention is, therefore, to specify materialsfor compression molds for optical elements which materials do not havethe disadvantages of the materials which are known for the applicationsmentioned and which permit substantially longer use as compression moldswhile retaining their respective geometrical spatial shapes underextreme physicochemical, thermal and mechanical process conditions.

This object is achieved according to the invention in that the oxideceramic material proposed is present in polycrystalline form. This canexpediently comprise aluminum oxide (Al₂ O₃) ceramic. It is alsopossible that it comprises a monophase zirconium dioxide (ZrO₂) and/orhafnium dioxide (HfO₂) ceramic. In addition, however, the material canalso be a multiphase ZrO₂ -- and/or HfO₂ --containing ceramic which iscomposed of a matrix component and at least one particle component whichis dispersed discretely in it. In a further embodiment of the presentinvention, the matrix component of the material can be a ceramic madefrom cubis and/or tetragonal ZrO₂ or HfO₂ or a (Zr,Hf)O₂ mixed crystalphase. It is also possible for the ceramic to comprise at least twomatrix components, for example tetragonal ZrO₂ and Al₂ O₃ and/orchromium oxide (Cr₂ O₃). According to a preferred embodiment of thepresent invention, an oxide ceramic material is used which comprises apolycrystalline matrix in which ZrO₂ and/or HfO₂ and/or (Zr,Hf)O₂crystallites are distributed homogeneously as discrete particles. Inthis case, these particles are present in polymorphous modification,arising in each case in their low-symmetry crystal lattice structure, atleast in the surface region of the material. Thus, a specific embodimentcomprises the particles in the surface region of the material having amonoclinic crystal lattice structure and those in the interior of thematerial having a tetragonal crystal lattice structure. It is expedientthat the particles have a size between 5 and 5,000 nanometers(nm)--preferably between 100 and 1,000 nm.

It is also possible for the polycrystalline matrix component to compriseat least one of the following substances: cubic ZrO₂, Al₂ O₃, Cr₂ O₃, anAl/Cr mixed oxide ((Al,Cr)₂ O₃) or a spinell (for example MgO.Al₂ O₃).The material intended for use according to the invention can furthermorecomprise cubic ArO₂ as the matrix component in which tetragonal ZrO₂and/or HfO₂ and/or (Zr,Hf)O₂ crystalline are distributed as discreteparticles. In this case, it is advantageous for the tetragonal particlesto be present in the matrix in coherent, crystallized form. According toa further embodiment of the present invention, the ceramic mayadditionally contain at least one of the partially-stabilized oxideslisted below (in each case in mol %): yttrium oxide (Y₂ O₃): 3.5 to 12;cerium oxide (CeO₂): 3.5 to 12; magnesium oxide (MgO): 6 to 16; calciumoxide (CaO): 6 to 16, and oxides of the rare earths (RE oxides): 3.5 to12. According to a subvariant, the ceramic here advantageously has aCeO₂ :ZrO₂ molar ratio between 8:92 and 30:70. In addition, it ispossible for the material to comprise exclusively at least one of thesubstances intended for the matrix component. It is also possible, inaddition, for up to 2% by weight of dopes from the group comprising theoxides: MgO, nickel oxide (NiO), tungsten oxide (WO₃) and/or molybdenumoxide (MoO₃) to be present. The density of the material intended for useaccording to the invention is at least 95%--preferably 100%--of thetheoretical density.

The advantages of the polycrystalline, mono- or polyphase oxide ceramicmaterial specified as a material for compression molds forpostworking-free shaping of elements made from glass or aglass--containing ceramic and having planar and/or spherical and/oraspherical convex and/or concave surfaces and having high surfacequality and dimensional accuracy are, firstly, that they have nomacroscopic anisotropy, in contrast to monocrystal materials, and,secondly, they have a markedly lower brittleness than correspondingmonocrystals. Thus, polycrystalline, dense Al₂ O₃ has a stress intensityfactor K_(ic) of 4-5 [MN .m⁻¹.5 ]; it is thus more than twice as greatas that of a corresponding moncrystal. The polycrystalline materialelement thus represents an important improvement for solution of thetechnical task mentioned.

In addition, special ceramics have been described and developed inrecent years which feature a particularly outstanding strengthbehaviour. For example, K_(ic) values of 17 Mnm⁻¹.5 have already beenmeasured. These are ZrO₂ --

and/or HfO₂ strengthened dispersion ceramics ("ZTC" ceramics:"Zirconia-Toughened Ceramics"). They also have, inter alia, an excellentthermal shock resistance, cf. Berichte der Deutschen KeramischenGesellschaft [Reports of the German Ceramics Society], Volume 55, No.11, 1978, 487-491.

Materials of the type mentioned may be monophasal, such as, for example,the "TZP" ("Tetragonal Zirconia Polycrystal") ceramics, cf. GermanOffenlegungschrift 3,408,096, or alternatively comprise several phases,such as the "PSZ" (Partially Stabilized Zirconia") ceramics, cf. Nature,Vol. 258, Dec. 25, 1975, 703-704. In addition, a conversion-toughenedaluminum oxide or corresponding ceramics, as are known, for example,from American Ceramic Society, Vol. 68, No. 1, January 1985, C4-C5.

The active oxide here is ZrO₂ or HfO₂ or an isomorphous mixed crystalform of both the starting components mentioned, which--if appropriatetogether with oxides such as CaO, MgO, Y₂ O₃, CeO₂ and RE oxides--areincorporated into the ceramic. The ZrO₂ -- and/or HfO₂ -- tougheneddispersion ceramics generally comprise a matrix in which ZrO₂ and/orHfO₂ are finely distributed--ie. dispersed. The matrix can be Al₂ O₃, acubic ZrO₂ phase, Cr₂ O₃ or, for example, a representative of thespinell group (cubic double oxide).

The dispersed, particulate ZrO₂ and/or HfO₂ phase exists in thetetragonal modification at the production temperature of ceramic. Below1,150° C., the tetragonal phase tends to be converted into thelow-symmetry, bulkier monoclinic phase. If the particles dispersed inthe matrix are very finely divided, the phase conversion can be"shifted" or "delayed" to below room temperature, since the matrixpressure "hinders" the new crystal lattice-structural orientation of theparticle components, which is connected with a volume increase.

If an extremely large pressure is exerted externally on thepolycrystalline material, so that a microcrack forms in the matrix, thiswill generally only be able to continue to a ZrO₂ or HfO₂ particle,where, as a consequence of the locally weakened matrix pressure, theconversion, hitherto prevented, into a low-symmetry, larger-volumemodification takes place. This lattice restructuring process consumesenergy, so that the consequence is an immediate slowing, or eventermination, of the microcrack propagation speed. In addition, the morebulky particles virtually "block" the corresponding microcracks. In thisfashion, the stress intensity factors and the strength values can beappreciably increased.

It was possible to achieve a further improvement in this respect byconstantly reducing the matrix phase. Through addition of CeO₂ and Y₂O₃, it has even been possible to develop a ceramic which comprisesexclusively a dispersed ZrO₂ phase. To a certain extent, a "matrix"pressure is achieved without an actual "matrix" in the conventionalsense being present. This ceramic is a so-called "TZP" (TetragonalZirconia Polycrystal") ceramic.

In addition to all the mechanical properties described above, all theceramics intended for use according to the invention have a positivesurface strengthening. This is caused by the fact that there is nomatrix pressure acting on all sides in the regions near the surface. Inthe proximity of the surface, the tetragonal particles are convertedinto the more bulky monoclinic modification, as early as duringproduction of the ceramic above room temperature, so that compressivestresses occur at the surface, leading to a further improvement in thematerial properties in the sense of a surface "toughening". Compressivestresses at the material surface can, in principle, be achieved bygrinding and polishing processes as as a consequence of local heating.This toughening mechanism is commonly called "stress-induced conversiontoughening".

However, the stress intensity factor K_(ic) can be appreciably increasedeven if the particles are coarser, so that they are converted duringtheir formation even before room temperature is reached. A network ofvery small microcracks is produced through the conversion of the nowcoarser particles. If energy is transferred elastically to the ceramicby an externally acting stress, the energy is distributed amongst alarge number of microcracks and not--as in normal polycrystalline ormonocrystalline, nonmetallic, inorganic, bodies--to a single microcrack.The toughening mechanism just described is called "microcracktoughening".

In addition to the material-specific advantages described here, use ofthe materials mentioned in the sense according to the invention causesdrastically increased economic efficiency, which is, on the one hand, aresult of the relatively high creep resistance and, on the other hand, aresult of significantly cheaper production of the materials compared tothe monocrystalline materials known hitherto. Finally, there are alsodimensioning problems for the compression molds, which, naturally, weredependent on the monocrystal volumes available in each case.

The compression molds can have a very wide variety of geometricalplanar, convex and/or concave shapes. They may be in one or severalparts. Of particular advantage for the application intended is thematerial property, discussed above, that, during shaping or cutting ofthe molds--ie. during creating of fresh surface (part)regions--precisely these zones are additionally surface-toughened as aconsequence of the reaction mechanism, the matrix/particle conformationand, above all, the crystallographic matrix/particle constitutions.

By means of known techniques using elevated temperature and/or with theaid of pressure, the polycrystalline ceramic material can be combined toform ceramic bodies having a density corresponding to the theoreticaldensity of the particular material.

We claim:
 1. The use of an oxide ceramic material for compression moldsfor shaping elements made from glass or a glass-containing ceramic andhaving high quality and dimensional accuracy, wherein the material ispresent in polycrystalline form and the matrix component thereofcomprises a ceramic made from one of a cubic and/or tetragonal HfO₂ or(Zr,Hf(O₂ mixed crystal phase.
 2. The use as claimed in claim 1, whereinthe material comprises a monophasal ZrO₂ -- and/or HfO₂ -- containingceramic.
 3. The use as claimed in claim 1, wherein the materialcomprises a multiphasal ZrO₂ -- and/or HfO₂ -- containing ceramic whichis composed of a matrix component and at least one particle componentdispersed discretely in the ceramic.
 4. The use as claimed in claim 1wherein the matrix component is a ceramic made from cubic and/ortetragonal ZrO₂.
 5. The use as claimed in claim 1 wherein the ceramiccomprises at least two matrix components.
 6. The use as claimed in claim5, wherein the matrix components comprise tetragaonal ZrO₂ and Al₂ O₃and/or Cr₂ O₃.
 7. The use as claimed in claim 1, wherein the materialcomprises a polycrystalline matrix in which ZrO₂ and/or HfO₂ and/or(Zr,Hf)O₂ crystallites are distributed homogeneously as discreteparticles.
 8. The use as claimed in claim 7, wherein the particles havepolymorphous modifications and are present in each case in theirlow-symmetry crystal lattice structure in the surface region of thematerial.
 9. The use as claimed in claim 8, wherein the particles in thesurface region of the material have a monoclinic crystal latticestructure and those in the interior of the material have a tetragonalcrystal lattice structure.
 10. The use as claimed in claim 7 wherein theparticles have a size between 5 and 5,000 nm--preferably between 100 and1,000 nm.
 11. The use as claimed in claim 1, wherein in thepolycrystalline matrix component comprises at least one of the followingadditional substances: Al₂ O₃, Cr₂ O₃, (Al,Cr)₂ O₃ and a spinell. 12.The use as claimed in claim 1, wherein the matrix component comprisescubic ZrO₂ in which tetragonal ZrO₂ and/or HfO₂ and/or (Zr,Hf)O₂crystallites are distributed as discrete particles.
 13. The use asclaimed in at least one of the previous claims, wherein the tetragonalZrO₂ and/or HfO₂ and/or (Zr,Hf)O₂ particles are present in the matrix incoherently crystallized form.
 14. The use as claimed in claim 1, whereinthe ceramic additionally contains at least one of the partiallystabilized oxides listed below (in mol %):

    ______________________________________                                        Y.sub.2 O.sub.3 :                                                                            3.5 to 12, preferably 8 to 10                                  CeO.sub.2 :    3.5 to 12, preferably 8 to 10                                  MgO:           6.0 to 16, preferably 8 to 14                                  CaO:           6.0 to 16, preferably 8 to 14                                  RE oxides:     3.5 to 12, preferably 8 to
 10.                                 ______________________________________                                    


15. The use as claimed in claim 1 wherein the ceramic has a CeO₂ :ZrO₂molar ratio between 8:92 and 30:70.
 16. The use as claimed in claim 1,wherein the material comprises exclusively at least one of thesubstances intended for the matrix component.
 17. The use as claimed inclaim 1, wherein up to 2% by weight of dopes from the group comprisingthe oxides MgO, NiO, WO₃ and/or MoO₃ are additionally present.
 18. Theuse as claimed in claim 1 wherein the material has a density of at least95%--preferably 100%--of the theoretical density.
 19. The use as claimedin claim 1 wherein the polycrystalline, monophasal or polyphasal oxideceramic material is intended as a material for a compression mold forreworking-free shaping of elements made from glass or a glass-containingceramic, having planar and/or spherical and/or aspherical convex and/orconcave surfaces and having a high surface quality and dimensionalaccuracy.